Broadly, the invention relates to an apparatus and method for measuring the energy change in a chemical reaction. More specifically, the invention is directed to an adiabatic reaction calorimeter for measuring heat energy, and a method of use.
The term calorimetry can be generally defined as measurement of energy in the form of heat. The science of calorimetry is widely used in the chemical industry to measure the quantity of heat which is liberated or absorbed in various processes, such as chemical reactions, changes of state, formation of solutions, and in determining heat capacities of substances. To cite a specific example, the chemical engineer usually needs to have a knowledge of the heat of reaction involved in a chemical process in order to properly calculate heat balances.
Over a period of years many different types of calorimeters have been developed for measuring heat energy. One of the conventional devices now in use is referred to as an adiabatic reaction calorimeter. In an adiabatic calorimeter the objective is to minimize heat transfer between the calorimeter and the surrounding structure and atmosphere.
A typical adiabatic calorimeter is used by the National Bureau of Standards, at Washington, D.C. This device is an adaptation of a heat capacity calorimeter, which employs a vacuum chamber and a radiation control shield. In this device a bomb vessel is suspended in the center of an evacuated cubical steel box designed to withstand a high vacuum. The bomb vessel is supported inside a cylindrical heat radiation shield, which has a heating (resistance) wire secured to the inner surfaces of the shield.
During a chemical reaction the temperature of the radiation shield is matched to the temperature of the bomb vessel by an electronic control system, which includes differential thermocouples on the bomb vessel and the shield. The purpose of the radiation shield and control system is to minimize heat transfer from the bomb vessel to the surrounding vacuum environment. A platinum resistance thermometer on the bomb vessel measures temperature change, and a calibration heater is also attached to the bomb vessel. The bomb vessel and radiation shield are attached to a rotatable shaft, which rotates the entire unit in opposite directions, to mix the contents of the bomb vessel during the reaction.
A typical experiment which can be carried out in this device is to determine the heat of combustion of an organic compound, such as phthalic acid. In the procedure the bomb vessel is charged with oxygen, at an operating pressure of about 500 p.s.i. The bomb vessel is provided with a resistance thermometer, calibration heater and electrodes for igniting a sample of the organic compound. The entire chamber (box) is evacuated with a high vacuum pump. After evacuation the temperature of the bomb vessel is adjusted and the radiation shield control is turned on. When the thermal (temperature) drift becomes constant, the organic sample is electrically ignited to initiate the combustion reaction.
As the temperature of the bomb vessel increases, the automatic shield control brings the shield up to the same temperature as the bomb. This is done to minimize heat loss, as explained above. Shortly after the sample is ignited, the bomb vessel is rotated, to wash down the walls of the vessel to insure a homogeneous solution. When the thermal drift again becomes constant, the reaction is complete, and the unit is disassembled. The system can be calibrated by adding a known quantity of electrical energy to the bomb vessel and by observing the temperature change, or by burning a compound, such as benzoic acid, which is designated as a standard by the National Bureau.
The adiabatic calorimeter device described above has several disadvantages. A major problem is that the device has a limited temperature range, since the steel box enclosure is not insulated. Another problem is that the device does not include separate vessels for keeping reactants separated prior to a reaction. This is an obvious drawback, since it is essential in many heat of reaction studies to keep the reactants separated until it is desired to observe the temperature change. Another problem is the complex construction of the device, which requires that the entire bomb and radiation shield assembly be rotated at the same time. This part of the device is not only a complex structure, but it is impractical for measuring energy change in a slow reaction.